1. Field of the Invention
The present invention relates to an output circuit, in particular, an output circuit capable of changing the voltage of an output terminal by the current inflow and outflow for the gate of an output transistor.
2. Description of Related Art
When a signal having sharp rising edges and falling edges is used in communications between different devices, the signal generates radiation noise and power-supply noise which may cause a malfunction in the system. To reduce these noises, it needs to decrease the slew rate of the rising edges and falling edges of the signal. Therefore, an output circuit for reducing the slew rate of an output signal with regard to the input signal has been proposed. These circuits are sometimes called slew rate circuits. Furthermore, in such circuits, the duty ratio of an output signal needs to be unchanged from the duty ratio of the input signal.
Japanese unexamined patent application publication No. 11-346147 (which is called “related art example” hereinafter) discloses an example of such circuits. FIG. 9 shows an output circuit 100 described in the related art example. The output circuit 100 controls the rising and falling of the gate voltage Vgate of an output transistor Qo by the current outputted from current sources Cs10, Cs11, Cs20, and Cs21. More specifically, when the gate voltage Vgate is changed from ground voltage to power supply voltage, both the current sources Cs10, Cs11 are used to increase the gate voltage Vgate until it reaches a certain voltage Vr1. Then, the current source Cs10 alone is used to increase the gate voltage Vgate after the gate voltage Vgate reaches the voltage Vr1. On the other hand, when the gate voltage Vgate is changed from power supply voltage to ground voltage, both the current sources Cs20, Cs21 are used to decrease the gate voltage Vgate until it reaches a certain voltage Vr2. Then, the current source Cs 20 alone is used to decrease the gate voltage Vgate after the gate voltage Vgate reaches the voltage Vr2.
As explained above, the output circuit 100 controls the transition time of the gate voltage Vgate by changing the number of current sources depending on the voltage value of the gate voltage Vgate. Using this control, the output circuit 100 makes the time difference between the rising edges of an input signal and an output signal substantially equal to the time difference between falling edges of the input signal and output signal. In this manner, the output circuit 100 reduces the slew rate of an output signal, and simultaneously enables substantially the same duty ratios between an input signal and an output signal.
However, the output circuit 100 controls the amount of current to the gate of an output transistor Qo depending on the gate voltage Vgate. Therefore, if noise gets into the output terminal, that noise is transmitted to the gate of the output transistor Qo through the parasitic capacitance between the drain and gate of the output transistor Qo. Then, the noise transmitted to the gate of the output transistor Qo may cause a malfunction in the comparators CP1 and CP2. That is, there is a problem that the output circuit 100 has low tolerance to noise transmitted from the output terminal.